Method and apparatus for reducing chemical reaction mechanisms

ABSTRACT

Method and apparatus for reducing chemical reaction mechanisms are disclosed. A method comprises obtaining data for one or more chemical species in a chemical reaction model from a database, the database including properties of the one or more chemical species; grouping the chemical species in a chemical reaction model into one or more isomer groups according to molecular properties of the chemical species; assigning a representative isomer to at least one isomer group; replacing, in one or more chemical reaction equations of the chemical reaction model, one or more groups of chemical species with a corresponding representative isomer; and executing the chemical reaction model by an apparatus to determine results.

RELATED APPLICATIONS

This application is a continuation of copending U.S. patent applicationSer. No. 13/446,839 filed Apr. 13, 2012, which claims the benefit under35 U.S.C. § 119(e) to U.S. Provisional Patent Application Ser. No.61/475,122, filed Apr. 13, 2011, which is incorporated by reference inits entirety and for all purposes.

TECHNICAL FIELD

The present application relates generally to chemical reactionsimulation models and, more particularly, to generating reduced reactionmechanisms.

BACKGROUND

Complex chemical reaction mechanisms involve large numbers ofinterrelated individual reactions and chemical species. The analysis ofcomplex chemical reactions under various conditions (i.e., modeling)requires solving numerous mathematical equations that represent theindividual species, taking into account reaction conditions of interest,reaction rate constants and the properties and quantities of variousindividual species. Reaction kinetics of the combustion process, inparticular, can involve thousands of discrete reactions and chemicalspecies. Analyzing complex chemical processes by solving the numerousrepresentative mathematical equations associated with detailed reactionmodels (mechanisms) consumes vast computing resources and requires longrun-times, especially over varied reaction conditions. Even relativelysmall increases in the number of chemical species involved may greatlyincrease computational storage requirements and processing, due in partto the increased number of calculations required to account forinterdependencies between the species equations.

Reaction mechanisms are a fundamental tool for accurately analyzingcomplex chemical reaction processes, such as combustion, for example. Amechanism includes data representing the reactions and species in achemical process, compiled in a way to allow mathematical simulation ofthe process to determine the chemical composition (state) of the systemat a given time or location in space. Such chemical composition (amountof each species or molecule in the system) may be expressed asrepresentative mathematical equations, or components thereof, that areoperable and solvable. Detailed multi-purpose mechanisms have beendeveloped through various means and methods to represent and simulatevarious types of chemical species and reactions, including by solvingassociated representative equations. However, very detailed reactionmechanisms are often of limited value in analyzing complex chemicalprocesses, because running simulations that include all the species insuch mechanisms, which are large enough to accurately capture the rangeof conditions in a modern combustion system, for example, is prohibitivein terms of computing time required. Generation of accurate reducedreaction mechanisms to represent complex chemical reactions requiressignificant effort and time. Increasing the efficiency and accuracy ofthe mechanism-reduction process facilitates more rapid and efficientresearch and development in the field of chemical reactions, especiallyfor combustion systems.

SUMMARY

Various aspects of examples of the invention are set out in the claims.

According to a first aspect of the present invention, a method comprisesaccessing data representing a master mechanism associated with achemical reaction model, identifying one or more chemical reactionequations and associated chemical species in the master mechanism,grouping the chemical species into one or more isomer groups accordingto molecular properties of the chemical species, assigning arepresentative isomer to each isomer group, and replacing each chemicalspecies that belongs to an isomer group and its properties with arepresentative isomer structure and associated representative propertiesthat correspond to the respective isomer group of the master mechanism.A reduced mechanism may thus be generated based on the master mechanism,for use in analyzing the chemical process. It is to be appreciated thatchemical species and reactions in a master mechanism may be expressed asrepresentative mathematical equations or components thereof, that areoperable and solvable.

A second aspect of the present invention provides a method forgenerating a reduced chemical reaction mechanism for a chemical reactionmodel. This method may be characterized by the following operations: (a)accessing data representing a master mechanism associated with achemical reaction model, the master mechanism including a set ofchemical reaction equations, a set of associated chemical reactionvariables, and a set of associated chemical species; (b) selectingtarget output variables from the set of chemical reaction variables; (c)assigning error tolerances to the target output variables; (d) solvingthe chemical reaction equations for the selected target output variablesand designating these solution values as master values; (e) performingan iterative isomer lumping process characterized by: (i) categorizingthe chemical species in the master mechanism into isomer groupsaccording to molecular structure and properties, by applying one or moreisomer lumping rules; (ii) replacing each chemical species categorizedas belonging to an isomer group, and its associated properties, with arepresentative isomer and representative set of properties to generate aprovisional mechanism; (iii) solving the chemical reaction equations forthe target output variables, using the provisional mechanism; (iv)calculating an error value for each target output variable, by comparingeach master value with the value of the corresponding target outputvariables found using the provisional mechanism; (v) applying asuccessive isomer lumping rule to replace more species withrepresentative isomers until the maximum error value for any targetoutput variable is greater than a predetermined threshold value; and (f)identifying the provisional mechanism with the least number of speciesresulting in all error values less than the predetermined thresholdvalue as the reduced mechanism.

In an aspect of the invention, the master mechanism may include chemicalreaction paths. Another embodiment may include an additional operationof defining a set of chemical reaction-flow models that representscertain operating conditions and in which the chemical reactionequations may be conservation equations that include variables andmathematical terms derived from the chemical reaction model. Suchchemical reaction equations and conservation equations may be expressedas mathematical equations that are operable and solvable. Target outputvariables may be selected from the set of reaction-flow modelconservation equations as described above, and the conservationequations may be solved to generate master values for these variables.The chemical species in these reaction-flow model conservation equationsmay be categorized into isomer groups and replaced with representativeisomers and properties and the conservation equations may be iterativelysolved, with successive species replacement by isomers, as discussedabove, to iteratively generate target output variables until the maximumerror value for any target output variable is greater than apredetermined threshold, as discussed above.

It is to be appreciated that the predetermined threshold value may beset as a numerical value, and that this predetermined threshold valuemay be set as 1 in embodiments of the invention. In other embodiments,at least one of the isomer lumping rules and the sequence of applyingthe rules may be predetermined. In still other embodiments, during thecourse of generating the reduced mechanism, one or more of the isomerlumping rules may be accessed from a database or other compilation ofisomer lumping rules, or the sequence of applying isomer lumping rulesmay vary depending on the outcome of each iteration.

Another aspect of the invention pertains to apparatus and computerprogram products, including machine-readable media on which programinstructions and/or arrangements of data for implementing the methodsdescribed above may be provided. For the performance of certain methodoperations, program instructions may be provided as computer code. Data,if employed to implement features of embodiments of this invention, maybe provided as data structures, database tables, data objects, or otherappropriate arrangements of specified information. Any of the methods ofthis invention may be represented, in whole or in part, as programinstructions and/or data provided on machine-readable media.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of example embodiments of the presentinvention, reference is now made to the following descriptions taken inconnection with the accompanying drawings in which:

FIG. 1 is a flow chart illustrating a method according to oneembodiment;

FIG. 2 is a flow chart illustrating a method according to anotherembodiment;

FIG. 3 5 illustrates an exemplary device within which the variousembodiments may be implemented.

DETAILED DESCRIPTION OF THE DRAWINGS

Example embodiments of the present invention and their potentialadvantages are understood by referring to FIGS. 1-3 of the drawings.

Even with an appropriate mechanism, however, analyzing the myriadequations in a complex chemical reaction model requires a large amountof time and computational resources, due to the sheer numbers andinterrelations of the reactions, species and conditions involved foreach simulation of the chemical reaction. Running repeated simulationsover varied reaction conditions requires even more time.

Several methods of chemical reaction mechanism reduction have evolved,to generate more manageable mechanisms for analysis, such as sensitivityanalysis, principal component analysis, detailed reduction, lumping, andadaptive reduction approaches for generating reduced mechanisms. Inparticular, species lumping is a mechanism reduction process thatgenerates reduced mechanisms with fewer distinct species than thepre-reduction mechanism. Accurate reduced mechanisms may be generated bya process of identifying and grouping together species that have similarmolecular structures and properties, and replacing, in the mechanism,each species that belongs to a particular group, with an isomer having aset of properties that is representative of the species in therespective group.

In accordance with the present invention, methods are presented forgenerating reduced mechanisms from master mechanisms representing largechemical reaction models. The reduced mechanisms generated by thesemethods include fewer distinct reaction species than the correspondingmaster mechanisms and thus require fewer calculations and less time torun a simulation of the chemical reaction model. In fact, thetime-savings achieved by employing accurate reduced mechanisms toanalyze chemical reaction models becomes increasingly pronounced whenthe analysis is performed over a wide range of conditions.

Although reducing the number of species in chemical reaction mechanismsreduces their complexity and the time required to solve the respectiveequations, this method of mechanism reduction also decreases theaccuracy of the reduced mechanism. A lumping approach characterizesspecies in a mechanism based on molecular structures and properties. Areduced mechanism is generated by analyzing chemical species in achemical reaction mechanism based on molecular structure and properties,lumping the species into isomer groups based on these characteristics,in light of a set of one or more isomer lumping rules, which may bepredetermined in whole or in part, and substituting representativeisomers and properties for corresponding species in the mechanismreaction equations. Since similar species are replaced by a singlerepresentative isomer, the overall number of distinct species in themechanism is reduced. In an embodiment, accuracy of the mechanismreduction process is controlled by an iterative process of applying anisomer lumping rule, from a set of rules, to generate a provisionalmechanism with a reduced number of species and modified chemicalreaction equations, solving the chemical reaction equations in theprovisional mechanism, evaluating the solutions to determine arespective error value, and, if the error values are within acceptablelimits, applying a successive isomer lumping rule, from the set ofrules, to generate another provisional mechanism with still fewerspecies. This iterative process is repeated until an error value fallsoutside acceptable limits. The provisional mechanism having the leastnumber of species for which the acceptable error limits are satisfied isthe reduced mechanism.

I. Standalone Mechanism Reduction Employing Isomer “Lumping.”

A feature of the methods described in connection with the invention is aprocedure for reducing chemical reaction mechanisms by grouping chemicalreaction species into isomer groups based on the molecular properties ofthe species, and replacing, in the chemical reactions, each speciesbelonging to a particular isomer group and its properties, with arepresentative isomer and a representative set of properties. Replacinga number of distinct chemical reaction species with a reduced number ofsubstituted isomers simplifies the number and complexity of calculationsrequired to run a chemical reaction model simulation.

Referring now to FIG. 1, a flow chart illustrates a method according toone embodiment. Various operations shown in FIG. 1 may be performed inthe order depicted or in some other order. In accordance with the methodof FIG. 1, a mechanism reduction process 10 begins at block 20, as datadescribing a master mechanism associated with a chemical reaction modelis accessed. The data may be accessed from a database of variouschemical species and their associated properties, for example. The dataincludes chemical reaction equations and chemical species representingchemical processes in the chemical reaction model, along with datarepresenting equation variables and constant values (e.g., thermodynamicrate constants), as well as the molecular structure, functional groups,and thermodynamic property data regarding the chemical species in themaster mechanism. The master mechanism may be a novel mechanism, or maybe a mechanism that has been used in previous chemical reaction modelingstudies, and may be accessed from a library, or generated as part of abroader chemical reaction model analysis that incorporates this process10 to reduce the master mechanism. In any case, the master mechanism andrelated data may be generated in accordance with principles and byvarious techniques. In addition, it is to be appreciated that chemicalspecies and reactions in a master mechanism may be expressed asrepresentative mathematical equations, or components thereof, that areoperable and solvable.

After the data describing the master mechanism is accessed (block 20),the operational parameters of the process 10 are set, as shown at block22. These parameters control subsequent steps of the process 10,particularly the analysis and categorization of species into isomergroups (blocks 24 and 26). As will be described in greater detail below,in one embodiment, setting operational parameters may include specifyingthe maximum difference in the values of chemical species thermodynamicproperties such as enthalpy and/or entropy that is acceptable among allchemical species categorized into an isomer group. Only species withproperties meeting this operational parameter will be included in acorresponding isomer group. Of course, in keeping with the invention,other thermodynamic properties can be used in a similar fashion forcategorizing species into isomer groups. As noted above, a database ofchemical species and their associated properties, such as thermodynamicproperties, may be used in this regard.

In an embodiment of the invention, the master mechanism may includechemical reaction paths. In yet another embodiment, specifying thereaction conditions as discussed above (block 22) may also includedefining a set of chemical reaction-flow models that represents certainoperating conditions and in which the chemical reaction equations of themaster mechanism may be conservation equations that include variablesand mathematical terms derived from the chemical reaction model. Suchchemical reaction equations and conservation equations may be expressedas mathematical equations that are operable and solvable. The chemicalspecies in these reaction-flow model conservation equations may becategorized into isomer groups and replaced with representative isomersand properties in the process of generating a reduced mechanism.

Once operational parameters are specified for process 10 (block 22),each chemical species' molecular structures and arrangements offunctional groups may be analyzed, as indicated at block 24. In anembodiment, this analysis may identify the number of carbon atoms in allof the straight chain of branched chain species, as well as the numberand positions of carbon-carbon atomic bonds and particular functionalgroups in each of the species, such as hydroxy, hydroxyl radical,carbonyl, peroxide, peroxy radical, and carbon radical groups.Similarities in the molecular structures and positioning of chemicalspecies' functional groups tend to reflect similarities in the chemicalspecies' properties for purposes of chemical reaction analysis,particularly regarding hydrocarbon species such as those in combustionreactions.

Thus, similarities between the structures of chemical species allow forthe substitution of representative isomers and representative propertiesfor similar species in chemical reaction equations, because the behaviorand properties of a single carefully selected representative isomer in areaction can approximate the behavior and properties of several similarchemical species. Generating reduced mechanisms by decreasing the numberof distinct chemical species, their respective properties, andassociated interrelations with other species in chemical reaction modelssimplifies the mechanism in question, thus facilitating fastersimulations and analysis.

After the molecular structure and properties of each chemical specieshas been analyzed and identified, the process categorizes, or groups,individual species into isomer groups based on similarities in thespecies' molecular structures and properties (block 26). Thiscategorization process may be referred to as “lumping.” In anembodiment, the lumping process may proceed in accordance with apredetermined series of lumping rules. It is to be appreciated that, inkeeping with the invention, other combinations of lumping rules may beapplied, or lumping rules may be applied in a different sequence thanset forth below, or that not all of the lumping rules set forth belowmust be applied. In other embodiments, the series of lumping rules maybe selected from a list or library containing such predetermined lumpingrule series.

As discussed earlier the following functional groups may be consideredin the lumping process: hydroxy, hydroxyl radical, carbonyl, peroxide,peroxy radical, and carbon radical groups. In an embodiment, species areconsidered to be candidates for isomers if they satisfy the followingisomer lumping rules:

-   -   They have the same number of each of the functional groups    -   Their main carbon chains (i.e. without considering the        attachments of all functional groups), including all carbon        branches, are identical    -   If the species are unsaturated species (i.e. alkenes), the        locations of carbon-carbon double or triple bond on the main        chains are identical

For analysis of low-temperature kinetic reactions, two additional rulesare implemented

-   -   1. The relative position of the carbon radical site to the        peroxy group is identical between the species for the following        conditions:        -   the carbon radical site is adjacent to the carbon site where            peroxy group is attached        -   the carbon radical site is exactly at 2nd position from the            carbon site where peroxy group is attached        -   the carbon radical site is exactly at 3rd position from the            carbon site where peroxy group is attached    -   2. The relative position of the carbon sites where the peroxy        radical group is attached is identical between the species for        the following conditions:        -   the two carbon sites are adjacent to each other        -   there is exactly one carbon atom between the two carbon            sites        -   there are exactly two carbon atoms between the two carbon            sites    -   The results on which these two additional rules are based, along        with additional details, can be found in S. S. Ahmed, F.        Mauss, G. Moreac, and T. Zeuch, “A Comprehensive and Compact        n-Heptane Oxidation Model Derived Using Chemical Lumping,” Phys.        Chem. Chem. Phys., 9: 1107-1126, 2007.

Analyzing chemical species and categorizing, or lumping, the speciesinto isomer groups by application of isomer lumping rules results in oneor more groups of chemical species with similar structures andproperties. The groupings may be recorded or stored for furtheranalysis.

At this point an additional lumping rule based on the values of standardenthalpy of formation of species may be applied: an isomer group ofspecies will be divided into sub-groups if the maximum difference instandard enthalpy of formation among the species is beyond a certainthreshold value. As discussed above, this threshold value may be set asan operational parameter as shown at block 22 of FIG. 1. As a result ofapplying this lumping rule, any group of species will exhibit adifference in standard enthalpy of formation within a pre-defined level.The discrepancy in this property between the lumped species andrepresentative isomers will thus be minimized, and the accuracy of thereduced mechanism will be improved. In an embodiment, a threshold valuefor maximum allowed difference in standard enthalpy of formation amonglumped species is 5 kcal/mole in order to generate the smallest lumpedmechanism while maintaining the highest level of accuracy.

Once chemical species are categorized into isomer groups based onlumping rules, each species in the isomer group is replaced, in thechemical reaction equations of the master mechanism, by a singlerepresentative isomer, as shown at block 28 of FIG. 1. This means thatall thermodynamic and transport properties of each species belonging toan isomer group are replaced by properties of the respectiverepresentative isomer. Accordingly, all chemical reaction equations inthe master mechanism that involve a species in a particular isomer groupare thus modified to include the respective representative isomer andthe representative properties. Data and properties corresponding torepresentative isomers may be accessed from a compilation or library ordatabase of representative isomers for inclusion in the equations of amechanism undergoing reduction. That said, other specific isomer dataand properties that rely on the number and types of species in theisomer group may be calculated as part of the mechanism reductionprocess, as described in greater detail, below.

Because each representative isomer replaces several distinct chemicalspecies, the thermodynamic and transport properties of a representativeisomer are combinations of the corresponding properties of each chemicalspecies in the isomer group. Various methods of accounting for thecontributions of each chemical species are known in the art, anddetailed descriptions of them are not central to the invention, and arebeyond the scope of this disclosure. In an embodiment of the invention,an equi-repartition approach is applied. This approach is known bypersons of skill in the art, and described in S. S. Ahmed, G. Moreac, T.Zeuch, and F. Mauss, “Reduced Mechanism for the Oxidation of theMixtures of n-Heptane and iso-Octane,” European Combustion Meeting,2005. This equi-repartition approach assumes that each chemical speciesin an isomer group makes the same contribution to the representativeisomer properties, so that the relative contribution of such a chemicalspecies is the reciprocal number of chemical species in the isomergroup. The process 10 can implement this process very simply, and theonly input required is the mechanism itself. Although theequi-repartition method may lead to reduced accuracy, as compared withanother method described in detail, below, the equi-repartition approachcomplements the standalone mechanism reduction process 10, because areduced mechanism can be generated utilizing an isomer lumping procedurethat does not require any inputs beyond the mechanism itself.

I.A. Adjustment of Rate Constants

Equations representing chemical reactions incorporate rate constantsthat quantify reaction speed. Since rate constants depend on theproperties of the species in the equation, replacement of chemicalspecies with isomers as part of a mechanism-reduction procedure alsorequires adjustment of rate constants in the reactions or equations, toaccount for the properties of the “replacement” representative isomer.The forward and reverse rate constants are related by the equilibriumconstants in the respective reaction equations. Although the specificdetails of the kinetic theory are beyond the scope of this disclosure,and are not required to understand the invention, in an embodiment ofthe invention, in an embodiment, three types of adjustments may be madefor the rate constants in the chemical reactions of the mechanism:

-   -   1. If the reaction represented by the particular reactions or        equation(s) in question is reversible, and the equilibrium        constant is used to calculate the reverse rate constant from the        forward rate constant, both the forward rate constant and the        equilibrium constant require adjustment.    -   2. If the reaction represented by the particular equation(s) in        question is reversible and the reverse rate constant is defined        explicitly, both the forward rate constant and the reverse rate        constant require adjustment.    -   3. If the reaction represented by the particular equation(s) in        question is irreversible, only the forward rate constant        requires adjustment.

For example, consider {A_(i)} and {B_(i)} as representing two isomergroups, and A_(i) and B_(i) represent a species from each group. L_(A)is the reciprocal of the relative contribution of A_(i) to the group{A_(i)} and L_(B) is the reciprocal of the relative contribution ofB_(i) to the group {B_(i)}. If the equi-repartition approach isutilized, L_(A) equals the number of isomers in the {A_(i)} group andL_(B) equals the number of isomers in the {B_(i)} group.

Species C and D do not belong to any group of isomers. If the reactionis reversible and the equilibrium constant is used to calculate thereverse rate constant from the forward rate constant, the reverse rateconstant is calculated using the following formula:

k _(r) =k _(f) /k _(eq)

-   -   The following adjustments will be applied to the forward rate        constant and the equilibrium constant:

A_(i) + C = D; k_(fnew) = k_(fold)/L_(A), k_(eqnew) = k_(eqold)/L_(A)A_(i) + B_(i) = C + D; k_(fnew) = k_(fold)/(L_(A)*L_(B)), k_(eqnew) =k_(eqold)/(L_(A)*L_(B)) D = A_(i) + C; k_(fnew) = k_(fold), k_(eqnew) =k_(gqold) *L_(A) C + D = A_(i) + B_(i); k_(fnew) = k_(fold), k_(eqnew) =k_(eqold) *(L_(A)*L_(B)) A_(i) + C = B_(i) + D k_(fnew) =k_(fold)/L_(A), k_(eqnew) = k_(eqold) *(L_(B)/L_(A))

-   -   If a reversible reaction in the mechanism is using an explicit        reverse rate constant, the following adjustment will be applied        to the forward rate constant and the reverse rate constant:

A_(i) + C = D; k_(fnew) = k_(fold)/L_(A), k_(rnew) = k_(rold) A_(i) +B_(i) = C + D; k_(fnew) = k_(fold)/(L_(A)*L_(B)), k_(rnew) = k_(rold) D= A_(i) + C; k_(fnew) = k_(fold), k_(rnew) = k_(rold)/L_(A) C + D =A_(i) + B_(i); k_(fnew) = k_(fold), k_(rnew) = k_(rold)/(L_(A)*L_(B))A_(i) + C = B_(i) + D; k_(fnew) = k_(fold)/L_(A), k_(rnew) =k_(rold)/L_(B)

-   -   For irreversible reactions, the following adjustment will be        applied to the forward rate constant:

A_(i) + C => D k_(fnew) = k_(fold)/L_(A) A_(i) + B_(i) => C + D k_(fnew)= k_(fold)/(L_(A)*L_(B)) D => A_(i) + C k_(fnew) = k_(fold) C + D =>A_(i) + B_(i) k_(fnew) = k_(fold) A_(i) + C => B_(i) + D k_(fnew) =k_(fold)/L_(A)

Once these calculations are performed and the values of the adjustedconstants are substituted for their corresponding rate constants in theequations, the equations may be solved.

After the isomer lumping rules are applied, representative isomers andrepresentative properties are substituted, in the chemical reactionequations of the master mechanism, for the chemical species in isomergroups. In addition, the equation constants are adjusted, and a reducedmechanism with fewer chemical species results and is in condition foruse in the analysis of chemical reaction models. The reduced mechanismmay be utilized immediately in analysis of the chemical reactions in thereduced mechanism itself, or incorporated into a larger process ofanalyzing a chemical reaction model by running simulations thatincorporate the reduced mechanism. Alternately, the reduced mechanismmay be recorded or stored for these and other uses in the future,whether by itself, or as part of a library of mechanisms. Particularmechanisms in such a library may be retrieved or accessed to facilitatechemical reaction analysis in conditions for which a particularmechanism in the library is appropriate.

Embodiments and methods of the invention may also be incorporated into alarger process of analyzing and simulating chemical reaction models. Forexample, a method for analyzing chemical reaction models may incorporatemechanism reduction methods of the present invention as a subroutinethat is run one or more times during the overall process. Even though areduced mechanism generated by the “standalone” mechanism reductionmethod described above may not have been optimized for accuracy, thebenefits of applying this method are twofold: the reduced mechanism maybe generated very rapidly; and the only input required is the mastermechanism. In contrast, more accurate reduced mechanisms may begenerated by following other methods described in this disclosure;although generation and analysis of these more accurate reducedmechanisms require more processing time, the generation of target outputvalues using the master mechanism, and the input of additionalinformation beyond the mechanism itself.

II. Mechanism Reduction Employing Iterative Isomer Lumping and ErrorTolerances.

Referring now to FIG. 2, a flowchart illustrates a method according toanother embodiment. Various operations shown in FIG. 2 may be performedin the order depicted or in some other order. Mechanism reductionprocess 200 begins by accessing data that describes a master mechanismassociated with a chemical reaction model (block 202). As discussedabove in connection with another embodiment, above, the data may beavailable from a database and may include a set of one or more chemicalreactions, a set of one or more chemical reaction variables and constantvalues (e.g., thermodynamic rate constants), along with datarepresenting the molecular structure, functional groups andthermodynamic properties of the chemical species in the mastermechanism. In addition, the data may include, among the chemicalreaction variables, a set of target output variables such as intrinsicsolution variables from a reaction model or equation, as well as derivedvariables (examples of such derived variables in combustion analysisinclude flame-speed, ignition-delay data, hear-release data and emissionindices). Also as discussed above, the master mechanism and related datamay be wholly or partially novel or previously developed, and may bewholly or partially accessed from a library, or generated as part of abroader chemical reaction model analysis that incorporates the mechanismreduction process 200.

In an alternative embodiment, a skeletal mechanism may be generated fromthe master mechanism prior to starting the mechanism reduction process,and the data corresponding to the skeletal mechanism may be accessed asshown at block 202. Methods such as the Directed Relation Graph method,for example, are useful in generating skeletal mechanisms. Such methodsare not central to this invention, and will not be discussed in detail.Skeletal mechanisms may include the same types of data as those found inmaster mechanisms (i.e., chemical reaction equations and chemicalspecies representing the chemical reactions in the chemical reactionmodel, along with data representing equation variables and constantvalues, such as thermodynamic rate constants, as well as the molecularstructure, functional groups, thermodynamic property data regarding thechemical species in the skeletal mechanism, and target outputvariables). Thus, data accessed as shown at block 202 may correspond toa master mechanism as described in the preceding paragraph, or askeletal mechanism derived from a master mechanism by employing variousmethods. In fact, several computational packages exist for thegeneration of mechanisms and skeletal mechanisms associated withchemical reactions.

After the data describing the master or skeletal mechanism is accessedat block 202, target output variables are selected as shown at block204. The selected target output variables correspond to the mechanismparameters of interest in the chemical reaction analysis. In anembodiment, the target output variables may be selected from apredetermined list or a library. Such a library may have been previouslygenerated in connection with developing the master or skeletal model fora chemical reaction model analysis.

In an embodiment, the target output variables may also be used inmonitoring and controlling the degree of accuracy of the mechanismreduction process 200. As will be discussed in greater detail below, themechanism reduction process 200 involves the removal of species frommechanisms. The removal of species, in turn, reduces the accuracy of themechanism, and results in a corresponding increase in a calculated errorvalue (error value calculation described in greater detail below). Theassignment of error tolerances to target output variables (block 206)facilitates control of the species removal procedure because, in anembodiment, no more species will be removed from a mechanism thanpermits compliance with the error tolerances. This control process andthe species removal procedure according to the invention are describedin greater detail, below.

In an embodiment, relative error tolerances and absolute errortolerances may be assigned to target output variables. For example,assigning an absolute error tolerance value of 1.E-6 mole fraction maybe appropriate for species fractions such as PPM of NOx. In thisexample, species removal would proceed for so long as the generatederror value is below this error tolerance value for the target outputvariable PPM of NOx. In another embodiment, all values of a targetoutput variable are to be considered when the respective absolute errortolerance is assigned a value of zero.

In addition to an absolute error value, a relative error tolerance maybe assigned to a target output variable. In an embodiment, a relativeerror tolerance value may be expressed as a percentage of a targetoutput variable predicted by the master mechanism. For example, if“Crank Angle at 10% of heat-release” (“CA10”) is selected as a targetoutput variable, and solving the master mechanism predicts a CA10 valueof approximately 7 degrees ATDC, setting a relative error tolerance of20% for this target output variable would result in species beingremoved from the mechanism until the CA10 value is within 1.4 degrees ofthe value predicted by the master mechanism.

An embodiment may provide various options for expressing error valuesgenerated in the error calculations described below. These options maybe provided in a list or menu, or selected from a library. For example,a generated error value may be expressed as the entire time/spatialprofile of the respective target output variable over the entire speciesremoval procedure. Alternately, the generated error value for aparticular target output variable may be expressed as only the end-pointvalue (“end point option”) or the maximum error value generated in anyiteration of the species removal procedure. In addition, forsingle-point target output variables such as ignition-delay time orflame speed, if the mechanism reduction process is performediteratively, the “end point option” will employ only the error valuegenerated from the final iteration, rather than employing valuesgenerated at each iteration of the process.

In embodiments of the present invention, operational parameters for themechanism can be specified. More specifically, once error tolerances areassigned as seen at block 206 of FIG. 2, certain species may bedesignated as exempt from the species removal procedures, as shown atblock 208. That is, in a mechanism reduction process 200, it may bedesirable to specify that certain species are not replaced with isomersin an isomer reduction procedure. For example, in a mechanism associatedwith a combustion reaction model, key species such as the inlet speciesand other species that are major combustion products may be designatedas exempt, so that they are not replaced by isomers in the reducedmechanism. The reduced mechanism would thus include these species andtheir properties rather than representative isomers and representativeproperties of isomer groups into which these species may otherwise havebeen categorized.

In another embodiment, if the target output variables selected at block204 are to be evaluated over a wide range of reaction conditions, asubset of reaction conditions can be specified for the reduction process200 (block 210), to simplify and make the mechanism reduction process200 more manageable. Reducing complexity in this way reduces therun-time needed to generate the reduced mechanism, and may result inimproved efficiency, particularly where the reaction conditions ofinterest are limited. Of course, the invention contemplatesspecification of other operational parameters for the mechanismreduction process (for example, reaction conditions or other reactionparameters, particularly when the mechanism reduction process isincorporated into a broader chemical reaction analysis).

In an embodiment of the invention, the master mechanism may includechemical reaction paths. In another embodiment, specifying the reactionconditions as discussed above (block 210) may also include defining aset of chemical reaction-flow models that represents certain operatingconditions and in which the chemical reaction equations of the mastermechanism may be conservation equations that include variables andmathematical terms derived from the chemical reaction model. Suchchemical reaction equations and conservation equations may be expressedas mathematical equations that are operable and solvable. In such anembodiment, target output variables may be selected from the set ofreaction-flow model conservation equations, and the conservationequations may be solved to generate master values for these variables.The chemical species in these reaction-flow model conservation equationsmay be categorized into isomer groups and replaced with representativeisomers and properties and the conservation equations may be iterativelysolved, with successive species replacement by isomers as discussed ingreater detail below, to iteratively generate target output variables inthe process of generating a reduced mechanism.

In order to control the mechanism reduction process through the use oferror tolerances, in another embodiment, solutions are generated for thetarget output variables of interest, by solving the equations in themaster mechanism for the selected target output variables. Thesesolution values may be designated, recorded or stored as “master values”(block 212).

Once the target output variables are selected (block 204), errortolerances are assigned (block 206), any operational parameters arespecified (blocks 208 and 210), and “master values” are generated (block212), the isomer replacement procedure commences (block 214). Thisprocedure, also known as isomer “lumping” includes aspects of the isomer“lumping” procedure described above, in connection with the “standalone”mechanism reduction process. That is, the lumping process categorizes,or “lumps,” individual chemical species into isomer groups based onsimilarities in the species' molecular structures and properties. Thecategorization follows a set of one or more lumping rules, one or moreof which may be predetermined, as set forth above in connection with the“standalone” mechanism reduction. In another embodiment, however, thelumping process may proceed in an iterative fashion, as shown at blocks214, 216, 218, 220, 222 and 224. This means that each lumping rule inthe set is applied in a cumulative, staged process, or in a discreteiteration, rather than at the same time that the other rules areapplied, so that species are removed in a gradual fashion at eachiteration, and the accuracy of the lumping process can be monitored andcontrolled. In other embodiments, the set of lumping rules to be appliedmay be wholly or partially selected from a list or library containing aset of lumping rules. In addition, the sequence of lumping rules may bepredetermined, or may be determined during the iterative lumping ruleapplication process, based on the interim results of the isomer lumpingprocess.

For example, once the target output variables have been selected (block204), error tolerances assigned (block 206), and any operationalparameters specified (blocks 208, 210), all chemical species in themechanism are analyzed and categorized based on based on similarities inthe species' molecular structures and properties (as discussed above,the following functional groups are considered in the lumping process:hydroxy, hydroxyl radical, carbonyl, peroxide, peroxy radical, andcarbon radical groups). Of course, the species designated as “exempt”from the lumping procedure at block 208 are not analyzed andcategorized. In keeping with the iterative nature of the process of thisembodiment, only a single (i.e., the “first”) isomer lumping rule isapplied at block 214 to categorize species. For example, the set ofisomer lumping rules described above, in connection with the“standalone” lumping method may be applied, albeit in an iterativefashion. That is, rather than applying all of the isomer rules prior togenerating a reduced mechanism, the first isomer lumping rule of the setis applied independently of the other rules, to generate a provisionalmechanism in which species having the same number of each of thefunctional groups are lumped into the same isomer group.

Thus, the species belonging to isomer groups based on the first lumpingrule are identified, and these species and their respective propertiesare replaced in the mechanism equations with representative isomers andrepresentative isomer properties corresponding to the respective isomergroup, as discussed above, to generate a provisional mechanism (block216). Replacing each species in a particular isomer group with a singlerepresentative isomer in the mechanism means that all thermodynamic andtransport properties of species belonging to an isomer group arereplaced by properties of the respective representative isomer.Accordingly, all chemical reaction equations in the mechanism thatinvolve a species in a particular isomer group are modified to includethe respective representative isomer and the representative properties.

Because each representative isomer replaces a group of distinct chemicalspecies, the thermodynamic and transport properties of a representativeisomer may be combinations of the corresponding properties of eachchemical species in the isomer group. Methods of accounting for thecontributions of each chemical species are known in the art. Forexample, in the description of the standalone mechanism reductionprocess described above, an equi-repartition approach is applied, asdescribed in S. S. Ahmed, G. Moreac, T. Zeuch, and F. Mauss, “ReducedMechanism for the Oxidation of the Mixtures of n-Heptane andiso-Octane,” European Combustion Meeting, 2005. This equi-repartitionapproach may be applied in an embodiment of an iterative isomerreplacement procedure in accordance with the teachings of the invention.

However, the assumption that all isomers contribute exactly the same allthe time can lead to lower accuracy of the lumped mechanism. Thus,another embodiment employs an alternative approach for accounting forthe contribution of the chemical species grouped into an isomer group.This approach is the constant ratio approach described and used in T.Lu, and C. K. Law, “Strategies for Mechanism Reduction for LargeHydrocarbons: n-Heptane,” Combust. Flame, 154: 153-163, 2008.

The constant ratio approach tracks the intra-group mass fraction of thespecies in each isomer group and uses the average mass fractions overall sampling points as the relative contribution of each species to thelumped species. It requires running simulations over the range ofconditions of interest, using the master mechanism to calculate theaverage of the intra-group mass fractions.

II.A. Adjustment of Rate Constants

As discussed above in connection with the standalone mechanism reductionprocess, the rate constants of chemical reaction equations in whichspecies have been replaced with representative isomers andrepresentative properties may need adjustment to account for this changein properties. Adjustment of the rate constants may proceed as describedin Section I.A., above. However, if the constant ratio approach isapplied rather than the equi-repartition approach, the followingadjustments to the chemical reaction equations must be made: in theconstant ratio approach, L_(A) equals the reciprocal of the averageintragroup mass fraction of A_(i) in the {A_(i)} group and L_(B) equalsthe reciprocal of the average intra-group mass fraction of B_(i) in the{B_(i)} group. Except for differences in the L_(A) and L_(B) values, therate constants for the constant ratio approach are adjusted in the samemanner as described in Section I.A., in connection with theequi-repartition approach. In an embodiment, the adjustment of rateconstant occurs in conjunction at each iteration of the process thatinvolves replacement of species with representative isomers (block 216).

Referring again to FIG. 2, the provisional mechanism is used to generatesolutions for the selected target output variables, as shown at block218. The solutions generated using the provisional mechanism arecompared with the corresponding master values generated using the mastermechanism, to generate a normalized error value for each target outputvariable (block 220). More specifically, the normalized error value foreach target output variable is determined by the following equation:

$\frac{{{MV} - {PV}}}{\left( {{R{{MV}}} + A} \right)}$

wherein:

-   -   MV is the master value corresponding to the target output        variable    -   PV is the value of the target output variable generated using        the provisional mechanism    -   R is the relative error tolerance assigned to the target output        variable    -   A is the absolute error tolerance assigned to the target output        variable

If the normalized error value for each target output variable is lessthan a predetermined threshold value, all assigned error tolerancesettings are satisfied. In an embodiment, this predetermined thresholdvalue may be 1.0. As shown at block 222, if all of the normalized errorvalues for the target output variables are below the predeterminedthreshold, another isomer lumping rule in the set will be applied (block224), and the isomer lumping procedure and rate constant adjustmentsteps will be repeated to generate a new provisional mechanism. At thispoint, more species will be replaced (block 216), solutions for targetoutput variables will be generated (block 218), and a new set ofnormalized error values will be generated (block 220) for comparisonwith the predetermined threshold value (block 222). Since, as discussedabove, removing more species from the mechanism increases the normalizederror values, applying isomer lumping rules in an iterative processresults in the replacement of more species with representative isomersand an increase the normalized error values at each successive step.

In one embodiment of the present invention the same series of isomerlumping rules as set forth in the above description regarding the“standalone” mechanism reduction process is applied, albeit in aniterative process. That is, each isomer lumping rule set forth above isapplied sequentially, and a new provisional mechanism is generated eachtime a successive isomer lumping rule is applied. In alternativeembodiments of the present invention, the lumping rules may be appliedin a different order than they appear in the series described above, orother lumping rules may be applied without departing from the inventiveconcepts. In addition, each provisional mechanism and the associatednormalized error values that are generated may be recorded or stored forlater access or retrieval.

The process of sequentially applying isomer lumping rules, generatingprovisional mechanisms and normalized error values and comparing thenormalized error values with a predetermined threshold value continues,as shown at blocks 216-224, until a normalized error value meets orexceeds the predetermined threshold value. At this point, the iterativeprocess terminates and the provisional mechanism with the fewest numberof species and in which no normalized error value exceeds thepredetermined threshold is designated, output or stored as a reducedmechanism (block 226). The process may also be terminated if all isomerlumping rules in a set are applied and no normalized error value greaterthan the threshold value is generated. In this case, the provisionalmechanism with the fewest isomers may be output, identified, designated,or stored as the reduced mechanism.

In embodiments of the invention, the iterative isomer reduction processmay be monitored by tracking the normalized error values as they aregenerated and the process may be stopped at any point. The mechanismreduction process according to the invention may also provide forselection and retrieval or access of any provisional mechanismsgenerated during the isomer lumping procedure, for use in modelingchemical reactions. This allows for great flexibility in analyzingchemical reaction models.

III. Computing Apparatus and Systems

It is understood that the various embodiments of the present inventionmay be implemented individually, or collectively, in devices comprisedof various hardware and/or software modules and components. Such adevice, for example, may comprise a processor, a memory unit, and aninterface that are communicatively connected to each other, and mayrange from desktop, server and/or laptop computers, to consumerelectronic devices such as mobile devices and the like. Such devices mayinclude input and peripheral devices, and other components that enablethe device to read and receive data and instructions from various media,input devices, a network, or other inputting means in accordance withthe various embodiments of the invention. It should be understood,however, that the scope of the present invention is not intended to belimited to one particular type of device.

As an example, FIG. 3 illustrates a block diagram of a device 300 withinwhich the various embodiments of the present invention may beimplemented. The device 300 comprises at least one processor 304 and/orcontroller, at least one memory unit 302 that is in communication withthe processor, and at least one communication unit 306 that enables theexchange of data and information, directly or indirectly, with acommunication medium, such as the Internet, or other networks, entitiesand devices. The processor 304 can execute program code that is, forexample, stored in the memory 302. The communication unit 306 mayprovide wired and/or wireless communication capabilities in accordancewith one or more communication protocols and interfaces, and thereforeit may comprise the proper transmitter/receiver antennas, circuitry andports, as well as the encoding/decoding capabilities that may benecessary for proper transmission and/or reception of data and otherinformation. The device 300 may further include various othercomponents, such as a display for providing output to the user and aninput interface (e.g., keyboard, mouse, etc.).

Similarly, the various components or sub-components within each moduleof the present invention may be implemented in software, hardware,and/or firmware. The connectivity between the modules and/or componentswithin the modules may be provided using any one of a variety ofconnectivity methods and media, including, but not limited to,communications over the Internet, wired, or wireless networks using theappropriate protocols.

IV. Chemical Reaction

The results from the model may be used to achieve desired results in achemical reaction. For example, a chemical reaction using the chemicalspecies lumped in the model mechanism may be caused to take place toachieve results that are substantially similar to those produced by thechemical reaction model using the lumping. In this regard, thetolerances used in the lumping can dictate the accuracy of the result.

Various embodiments described herein are described in the generalcontext of method steps or processes, which may be implemented in oneembodiment by a computer program product or module, embodied in acomputer-readable memory, including computer-executable instructions,such as program code, and executed by apparatus such as computers orcomputing systems in networked environments. A computer-readable memorymay include removable and non-removable storage devices including, butnot limited to, Read Only Memory (ROM), Random Access Memory (RAM),compact discs (CDs), digital versatile discs (DVD), etc. As such, thevarious disclosed embodiments can be implemented by computer codeembodied on non-transitory computer readable media. In other embodimentsprocesses may be employed to perform operations on data, wherein theinstructions for process operations and the data, or elements thereof,may reside on or be transferred through one or more computing devices orsystems.

Embodiments of the present invention may be implemented in software,hardware, application logic or a combination of software, hardware andapplication logic. The software, application logic and/or hardware mayreside on a client device, a server or a network component. If desired,part of the software, application logic and/or hardware may reside on aclient device, part of the software, application logic and/or hardwaremay reside on a server, and part of the software, application logicand/or hardware may reside on a network component. In an exampleembodiment, the application logic, software or an instruction set ismaintained on any one of various conventional computer-readable media.In the context of this document, a “computer-readable medium” may be anymedia or means that can contain, store, communicate, propagate ortransport the instructions for use by or in connection with aninstruction execution system, apparatus, or device, such as a computer,with one example of such a device described and depicted in FIG. 3. Acomputer-readable medium may comprise a computer-readable storage mediumthat may be any media or means that can contain or store theinstructions for use by or in connection with an instruction executionsystem, apparatus, or device, such as a computer. In one embodiment, thecomputer-readable storage medium is a non-transitory storage medium.

In various respects, embodiments of the present invention my use datafrom a database. For example, a database of chemical species may beaccessed to obtain various properties of the chemical species. Otherdata described in this disclosure may be similarly obtained from orstored in a database.

If desired, the different functions discussed herein may be performed ina different order and/or concurrently with each other. Furthermore, ifdesired, one or more of the above-described functions may be optional ormay be combined.

Although various aspects of the invention are set out in the independentclaims, other aspects of the invention comprise other combinations offeatures from the described embodiments and/or the dependent claims withthe features of the independent claims, and not solely the combinationsexplicitly set out in the claims.

The foregoing description of embodiments has been presented for purposesof illustration and description. The foregoing description is notintended to be exhaustive or to limit embodiments of the presentinvention to the precise form disclosed, and modifications andvariations are possible in light of the above teachings or may beacquired from the practice of various embodiments. The embodimentsdiscussed herein were chosen and described in order to explain theprinciples and the nature of various embodiments and its practicalapplication to enable one skilled in the art to utilize the presentinvention in various embodiments and with various modifications as aresuited to the particular use contemplated. The features of theembodiments described herein may be combined in all possiblecombinations of methods, apparatus, modules, systems, and computerprogram products.

What is claimed:
 1. A processor implemented method for a chemicalreaction model of a physical system, the method comprising: obtainingdata for two or more chemical species in a chemical reaction model forthe physical system, wherein the chemical reaction model includes datarepresenting reactions and the two or more chemical species based onmathematical equations in a chemical process, the chemical reactionmodel executable by an apparatus to determine results of chemicalreactions of the physical system; grouping the two or more chemicalspecies in the chemical reaction model into one or more isomer groupsaccording to molecular properties of the two or more chemical species inthe data; generating a second chemical reaction model based on thechemical reaction model, wherein the second chemical reaction modelincludes a particular one of the one or more isomer groups to representa plurality of chemical species of the particular one isomer group; andexecuting the second chemical reaction model by the apparatus todetermine the results-of the chemical reactions of the physical systemwithout executing the chemical reaction model.
 2. The method of claim 1,further comprising: causing a reaction corresponding to a chemicalreaction of the physical system to take place using the two or morechemical species based on the chemical reaction model.
 3. The method ofclaim 1, wherein the operation of grouping the two or more chemicalspecies into one or more isomer groups further comprises: specifying anallowable difference of a thermodynamic property among one or morechemical species in a particular isomer group; and dividing theparticular isomer group into sub-groups if the allowable difference isexceeded.
 4. The method of claim 1, wherein the two or more chemicalspecies are grouped into the one or more isomer groups based on one ormore lumping rules.
 5. The method of claim 4, wherein the two or morechemical species are sorted based on molecular properties according toone of the one or more lumping rules.
 6. The method of claim 4, whereinat least one of the one or more lumping rules is predetermined.
 7. Themethod of claim 1, wherein representative isomer properties of an isomergroup are calculated as a function of the relative contribution of eachchemical species in the isomer group.
 8. The method of claim 1, whereinone of the mathematical equations is based on a rate value and anequilibrium value, and wherein the second chemical reaction model isgenerated based on adjusting the equilibrium value and the rate valuefor the particular one isomer group.
 9. The method of claim 1, whereinthe second chemical reaction model includes fewer chemical species thanthe chemical reaction mode.
 10. An apparatus for determining results ofchemical reactions of a physical system, the apparatus comprising: atleast one processor; and at least one memory including computer programcode, the at least one memory and the computer program code configuredto, with the at least one processor, cause the apparatus to perform:obtaining data for two or more chemical species in the chemical reactionmodel, wherein the chemical reaction model includes data representingreactions and the two or more chemical species based on mathematicalequations in a chemical process, the chemical reaction model executableto determine the results for the physical system; grouping the two ormore chemical species in the chemical reaction model into one or moreisomer groups according to molecular properties of the two or morechemical species; generating a second chemical reaction model based onthe chemical reaction model, wherein the second chemical reaction modelincludes a particular one of the one or more isomer groups to representa plurality of chemical species of the particular one isomer group; andexecuting the second chemical reaction model to determine the resultswithout executing the chemical reaction model.
 11. The apparatus ofclaim 10, wherein the apparatus further performs the step of: causing areaction corresponding to a chemical process of the physical system totake place using the one or more chemical species based on the chemicalreaction model.
 12. The apparatus of claim 10, wherein an allowablethreshold is specified for differences of a thermodynamic property amongone or more chemical species in a particular isomer group, wherein theoperation of grouping the chemical species into one or more isomergroups further comprises: dividing the particular isomer group intosub-groups if the allowable difference is exceeded.
 13. The apparatus ofclaim 10, wherein the two or more chemical species are grouped into theone or more isomer groups based on one or more lumping rules.
 14. Theapparatus of claim 13, wherein the two or more chemical species aresorted based on molecular properties according to one of the one or morelumping rules.
 15. The apparatus of claim 13, wherein at least one ofthe one or more lumping rules is predetermined.
 16. The apparatus ofclaim 10, wherein representative isomer properties are calculated as afunction of the relative contribution of each chemical species in anisomer group.
 17. The apparatus of claim 10, wherein one of themathematical equations is based a rate value and an equilibrium value,and wherein the second chemical reaction model is generated based on anadjustment of the equilibrium value and the rate value for theparticular one isomer group.
 18. The apparatus of claim 10, wherein thesecond chemical reaction model includes fewer chemical species than thechemical reaction model.
 19. A non-transitory computer-readable mediumstoring computer instructions that when executed by one or moreprocessors cause the one or more processors to perform: obtaining datafor two or more chemical species in a chemical reaction model for thephysical system, wherein the chemical reaction model includes datarepresenting reactions and the two or more chemical species based onmathematical equations in a chemical process, the chemical reactionmodel executable by an apparatus to determine results of chemicalreactions of the physical system; grouping the two or more chemicalspecies in the chemical reaction model into one or more isomer groupsaccording to molecular properties of the two or more chemical species inthe data; generating a second chemical reaction model based on thechemical reaction model, wherein the second chemical reaction modelincludes a particular one of the one or more isomer groups to representa plurality of chemical species of the particular one isomer group; andexecuting the second chemical reaction model by the apparatus todetermine the results-of the chemical reactions of the physical systemwithout executing the chemical reaction model.
 20. The medium of claim19, wherein the second chemical reaction model includes fewer chemicalspecies than the chemical reaction model.